1. Field of the Invention
The present invention relates to an electrophotographic machine, and, more particularly, to a method of aligning a laser printhead in an electrophotographic machine such as a tandem color laser printer.
2. Description of the Related Art
An electrophotographic machine such as a tandem color laser printer requires four individual laser scanner printheads to independently and simultaneously image a specific color on each of four respective photoconductive drums. In an in-line color electrophotographic imaging process, latent images are formed on the photosensitive drums, and the images are in turn developed using a predetermined color of toner. All four of these toner images are then transferred simultaneously onto a transfer medium, such as a belt. The developed images are then transferred to a sheet of media (such as paper) which travels past the transfer medium. The image in each color is created one line at a time, and the lines are oriented at right angles to the direction of travel of the sheet of media. The individually generated images combine to form a full-color image. In a typical multi-color laser printer, for example, the transfer medium passes through four color developing stations in series, with the colors being yellow, cyan, magenta and black.
It is recognized that in order for the multi-color laser printer to print accurately, the laser beams for all four colors must be in alignment, both in the scan direction (across the page) and the process direction (feed direction of the print medium). However, providing proper alignment of even a single laser printhead in relation to the sheet of media in the process direction can be difficult. This problem is compounded with the addition of each printhead, since the plurality of printheads must be in registration so that the individual images generated by each printhead can be superimposed correctly when combined. During printer assembly an attempt is made to optically align the laser printheads both individually and collectively, but the ability to provide precise alignment is limited by several factors, including component tolerances.
What is needed in the art is a method of quickly and accurately achieving skew adjustment and margin alignment in an electrophotographic machine.
The present invention provides a fixture to achieve the skew adjustment and the margin alignment of all four color planes to the accuracy required for high quality color printing with minimum operator interaction in a production environment.
The invention comprises, in one form thereof, a method of aligning a laser printhead in an electrophotographic machine having a plurality of photoconductive drums. A first of the photoconductive drums is replaced with a drum fixture containing a pair of fixed sensors located where the image plane of the laser beam would intersect the drum. The fixed sensors are spaced apart along a length of the first photoconductive drum axis. Each of the fixed sensors is fixed relative to a second of the photoconductive drum axes. The first photoconductive drum fixture is also provided with a pair of floating sensors. The floating sensors are spaced apart along the length of the first drum fixture. Each of the floating sensors is fixed relative to the first photoconductive drum fixture, but this photoconductive drum fixture floats relative to the second photoconductive drum axis. A laser beam from the laser printhead is scanned across an outside surface of the first drum fixture, the fixed sensors and the floating sensors. An intersection of the scanned laser beam and the outside surface of the first drum defines a scan path. A first skew of the scan path relative to an axis of the first photoconductive drum is measured using the floating sensors. A second skew of the scan path relative to an axis of the second photoconductive drum is measured using the fixed sensors. The scan path of the laser beam is adjusted dependent upon each of the first skew and the second skew.
The invention comprises, in another form thereof, an electrophotographic machine including a plurality of photoconductive drums. A laser printhead scans a laser beam across a fixture in the location of a first of the photoconductive drums. A pair of fixed sensors are spaced apart along a length of the first photoconductive drum axis. The fixed sensors are fixed relative to a second of the photoconductive drum axes. The fixed sensors sense a skew of the laser beam relative to the second photoconductive drum axis. A pair of floating sensors are spaced apart along the length of the first photoconductive drum fixture. The floating sensors are fixed relative to the first photoconductive drum fixture. The floating sensors mounted to the first drum fixture sense a skew of the laser beam relative to the first photoconductive drum axis. A processing/feedback unit is in communication with the fixed sensors and with the floating sensors. The processing/feedback unit calculates a desired skew of the laser beam and provides an indication of a desired skew target.
The assembly and alignment of a tandem color laser printer requires that the line created by the intersection of the plane of the laser scan and the surface of the photoconductive drum cause the image when transferred to the transfer medium to be parallel to the image of the reference photoconductive drum (usually the black drum). This reference photoconductive drum (black) datum is chosen as the datum for parallel transferred images. Thus, the black laser scan is aligned parallel to this black reference photoconductive drum datum axis just as is done in a single color (mono) laser printer. The other laser scans are aligned skewed with their respective photoconductive drum datum axis in an amount equal to the skew between their respective photoconductive drum datum axis and the black reference photoconductive drum datum axis. Thus, each of the non-black color laser scans will be adjusted to have twice the skew relative to the black laser scan (or to the black reference photoconductive drum datum axis) that its respective color photoconductive drum datum axis has compared to the black reference photoconductive drum datum axis. This implies that the skew in each photoconductive drum datum axis relative to the black reference photoconductive drum datum axis must be measured with the adjustment fixture of the present invention. The adjustment fixture must also measure the skew in the respective laser scan relative to its respective photoconductive drum datum axis.
It is desired to adjust the skew of the image generated by each printhead relative to the black reference photoconductive drum datum axis to within 0.015 mm over the 215.9 mm (8.5 inch) writing line length. This is achieved by adjusting the mechanical position of the printhead relative to its respective photoconductive drum datum axis during the assembly process. The printhead is designed with a coarse adjustment and a fine adjustment feature that can allow this precise skew adjustment to be achieved. The assembly operator receives easy to use feedback which provides the required target to achieve the desired skew adjustment and the instantaneous status of the laser scan relative to this desired target.
The assembly and alignment of a tandem color laser printer also requires aligning other registration characteristics of the four transferred images. The adjustment fixture of the present invention provides data to the printer that allows the printer to electronically adjust the left-right margin locations for all four colors to an equal nominal location, adjust the line lengths of all four colors to be equal, and adjust the relative timing of imaging each color to correct for the process direction spacing of each photoconductive drum datum axis relative to the black photoconductive drum datum axis. These three settings are then stored in the printer NVRAM ready for customer use.
An advantage of the present invention is that, in a production environment, an assembly operator can quickly achieve the skew adjustment and margin alignment of all four color planes that is required for high quality color printing.